Magnetic stimulation device

ABSTRACT

A magnetic stimulation apparatus for triggering action potentials, particularly in more deeply disposed, neuro-muscular tissue of a patient as well, has at least one stimulation coil that has terminals connected to the output of a current-generating unit offers greater degrees of freedom in the selection of the current pulse shapes because the current-generating unit provides current pulses generated in non-resonant fashion for the stimulation coil.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a magnetic stimulation apparatus ofthe type having at least one stimulation coil that has terminalsconnected to the output of a current-generating unit.

2. Description of the Prior Art

Magnetic stimulation apparatuses serve for magnetic stimulation of nervefibers and muscle tissue in the field of medical diagnostics andtherapy. Compared to electrical stimulation with stimulation current,the advantage of magnetic pulse stimulation lies in the lower pain ofthe stimulation, since no higher current densities in the region of thepain receptors of the skin occur given magnetic pulse stimulation. Afurther advantage of magnetic stimulation lies in the higher penetrationcapability, as a result whereof the excitation of tissue lying deeper isalso possible, particularly nerve fibers that lie deeper.

The book by R. F. Schmidt (editor), “Neuro-und Sinnesphysiologie”,Springer, second, amended edition 1995, Chapters 2 and 3, contains anexact description of neuro-physiological occurrences. The nerve system,for example, coordinates the activities of the various organs andreactions of the body to the environment. This mainly occurs due tomodifications of the potential of nerve cells. All cells have aquiescent potential. At the quiescent potential, all membrane currentsof a cell are in equilibrium. When the membrane potential is depolarizedby an additional membrane current that, for example, proceeds into thecell due to an external influence, then this is accompanied by a changein potential, referred to as an action potential. The aforementioned,depolarizing membrane current is also called stimulus. The triggerpotential for an action potential is called threshold. The equilibriumof the membrane currents changes at threshold. Additional membranecurrents that depolarize the membrane occur for a short time. Thiscondition is also called excitation. An action accompanies an actionpotential. Thus, for example, each spasm of a muscle fiber isaccompanied by an action potential in the muscle fiber, and eachreaction of a sensory cell to a sensory stimulation is accompanied byaction potentials.

European Application 0 182 160 discloses an apparatus for generatingelectromagnetic pulses with a semicircular shape and a frequency of 100Hz that, in particular, serves for promoting the micro-circulation ofthe blood in the region of the hair roots and the skin, for example toprevent hair loss. To that end, a diode rectifier bridge in Graetzcircuitry that feeds a pulse-generating coil is connected to an A.C.voltage transformer.

German OS 36 07 235 discloses an apparatus for generating unipolar airions and electromagnetic pulse fields for reducing the human reactiontime while simultaneously increasing the attention readiness. Forgenerating the electromagnetic pulse fields, a frequency generator thatgenerates a frequency in the range from 8 Hz through 10 Hz and having afollowing out-coupling amplifier and a coil generating the pulse fieldis connected to a voltage source.

German OS 41 32 428 discloses a magneto-therapy apparatus formagneto-therapeutic treatment. For generating a pulse setting magneticfield with a pulse repetition rate between 0.25 Hz and 2 Hz, an unstablemulti-vibrator is connected to a battery, this multi-vibrator feedingtwo cylindrical coils filled with iron. The apparatus is fashioned as apocket apparatus.

U.S. Pat. No. 5,743,844 discloses an apparatus for therapy withpulsating electromagnetic fields for promoting healing of bone and bodytissue, particularly in an embodiment as a battery-powered apparatusthat can be worn on the body. To that end, a coil generating themagnetic field is supplied from two voltage sources of different voltageheight via a specific circuit that contains two field effect transistorsand two capacitors as critical elements. The aforementioned circuitthereby has a fixed pulse-to-pause time relationship.

The devices disclosed in the above-cited documents are all designed suchthat the magnetic pulsed or, alternating fields they generate act on thehuman body below the threshold for triggering action potentials. Theeffects in the human body that can thus actually be achieved are partlyvery diffuse and scientifically controversial. Magnetic stimulationdevices that intentionally trigger action potentials, particularly inmore deeply disposed neural-muscular tissue, however, constitute anentirely different category of devices. Not only are the use andtherapeutic effect of these devices different but they operate atmultiply higher electrical powers to be provided, this being reflectedin correspondingly high current and voltage values. The devicesdisclosed in the above-cited documents are not suitable for this purposedue to their overall low-voltage and micro-current-oriented design.

A magnetic stimulation apparatus for triggering action potentials, inmore deeply disposed, neuro-muscular tissue as well, is described in thearticle by M. Schmid, T. Weyh and B. -U. Meyer, “Entwicklung,Optimiereung und Erprobung neuer Geräte für die magnetomotorischeStimulation von Nervenfasern”, Biomedizinische Technik, 38 (1993), pages317 through 324. It comprises a stimulation coil to which current pulsesgenerated in resonant fashion are supplied. The current-generatingcircuitry required for generating the current pulses comprises acontrollable power pack part as well as a high-voltage capacitor thatforms a parallel resonant circuit together with the stimulation coil,i.e. it operates as a resonant circuit. The high-voltage capacitor ischarged by the controllable power pack part and thereby accumulates thepulse energy required for the output of a current pulse.

The resonant frequency of the parallel resonant circuit formed by thestimulation coil and the high-voltage capacitor is defined by theselection of the capacitance of the high-voltage capacitor and by theinductivity of the stimulation coil and lies in the range from 1 through3 kHz. When the capacitance of the high voltage capacitor is varied,then the resonant frequency of the parallel resonant circuit and, thus,the rate of the current rise in the stimulation coil can be modified.The stimulus intensity is defined by the initial voltage at thehigh-voltage capacitor. Only the repetition rate, which lies in the areaaround 10 Hz, can be set as a further parameter.

Further, German OS 196 07 704 A1 discloses an apparatus for magneticexcitation of neural-muscular tissue. The known apparatus comprises anexcitation coil (stimulation coil) that forms a parallel resonantcircuit together with a storage capacitor (high-voltage capacitor), i.e.likewise works as 5 resonant circuit. Given this apparatus as well,resonant frequencies can only be realized in the range from 1 through 3kHz.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to create a magneticstimulation apparatus for triggering action potentials in more deeplydisposed neural-muscular tissue as well that offers greater degrees offreedom in the selection of the current pulse shapes.

The above object is achieved in an inventive magnetic stimulationapparatus having at least one stimulation coil that has terminalsconnected to the output of at least one current-generating unit, wherebythe current-generating unit offers non-resonantly generated currentpulses for the stimulation coil.

By abandoning resonant operation, greater degrees of freedom can beachieved in the selection of the current pulse shapes. Moreover, noregulatable power pack parts having specific charging circuits arerequired.

Since the stimulation coil—by contrast to the comparable magneticstimulation devices of the prior art—is not part of a parallel resonantcircuit, further degrees of freedom derive due to the selection of theinductivity of the stimulation coil.

In an embodiment the current-generating unit includes at least onecontrollable power converter having at least one power semiconductorswitch with short switching times that can be switched on and off.

As used herein the term “short switching time” means switching times ofapproximately 1 μs or less.

The power semiconductor switches that can be switched on and off andthat are provided in the aforementioned embodiment must exhibit shortswitching times, so that transistors, particularly IGBTs or MOSFETs, arepreferably currently employed therefor.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a magnetic stimulation apparatusconstructed and operating in accordance with the principles of thepresent invention.

FIG. 2 shows a curve of current pulses that are generated innon-resonant fashion in an embodiment of the inventive magneticstimulation apparatus, a voltage curve corresponding thereto which isproduced within the current-generating unit of the inventive apparatus,and current and voltage curves that are generated by an apparatusaccording to the prior art.

FIG. 3 shows a curve of the voltage at the stimulation coil thatcorresponds to the curve of the current pulses of FIG. 2 generated bythe inventive stimulation apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an embodiment of an inventive magnetic stimulationapparatus. The apparatus includes at least one stimulation coil L_(s)that has terminals connected to the output of a current-generating unit9. The current-generating unit 9 is formed, for example, by the parallelcircuit of a power pack 6, which need not be controllable and ispreferably implemented as high-voltage power pack part, an intermediatevoltage circuit 5 and a controllable power converter 8 that contains atleast one power semiconductor switch T₁ with short switching times thatcan be switched on and off. The intermediate voltage circuit includes atleast one intermediate circuit capacitor C_(z) and is charged by thepower pack part.

Given the magnetic stimulation apparatus shown in FIG. 1, thestimulation coil is supplied with the energy stored in the intermediatecircuit capacitor after the power semiconductor switch is turned on anda current pulse is thus triggered. After the power semiconductor switchis turned off, the current flowing in the stimulation coil is dismantledvia an unbiased branch 7 arranged in the controllable power converter.At least one unbiased diode D_(F) is arranged in the unbiased branch. Inthe illustrated exemplary embodiment, a resistor R_(F) is connected inseries with the unbiased diode D_(F) in the unbiased branch. Theunbiased resistor can also be omitted if the parasitic resistances inthe circuit are adequately high.

In FIG. 2, curve 1 is the curve of the output voltage of a high-voltagecapacitor given a current-generating unit in a magnetic stimulationapparatus according to the prior art. The high-voltage capacitor, whichforms a parallel resonant circuit with the stimulation coil, is chargedby a controllable power pack and thereby accumulates energy required forthe output of a current pulse.

Curve 2 is the curve from an output voltage of an intermediate circuitcapacitor in an intermediate voltage circuit of an inventive magneticstimulation apparatus. The intermediate circuit capacitor C_(Z) ischarged by the power pack 6, which need not be controllable, andsupplies voltage to the controllable power converter 8 when itdischarges.

The output voltage curve 1 as well as the output voltage curve 2 areshown referenced to the peak value U/Û of the voltage. The maximumvalues of the output voltage curves 1 and 2 are thus equal to one.

Given the magnetic stimulation apparatus according to the prior art, thedischarge of the high-voltage capacitor leads, according to the voltagecurve 1, to a current pulse (reference 3) through the stimulation coil.

The current pulse represented by curve 3 rises during the discharge ofthe high-voltage capacitor in such a prior art stimulator, lasting 150μs, up to an amplitude value of 9 kA (voltage curve 1). When theamplitude value of 9 kA is reached, the voltage has dropped to zero.Since energy is no longer resupplied by the high-voltage capacitor, thecurrent pulse decays within a decay time of approximately 150-200 μs.After a renewed charging of the high-voltage capacitor in such a priorart stimulator, the above-described, resonant generation of a currentpulse begins anew. The current pulse 3 generated in resonant fashion andshown in FIG. 2 thus has a pulse width of 150 μs plus decay time.

Compared thereto, the output voltage 2 across the intermediate circuitcapacitor C_(Z) given the inventive magnetic stimulation apparatusremains constant, since the current pulses shown as curve 4 in FIG. 2are produced in a non-resonant fashion according to the invention. Giventhe inventive pulsing, alternating current pulses 4 are generated invery rapid succession. The current pulses of curve 4 shown in FIG. 2exhibit, for example, a current amplitude of 1.5 kA and a pulse width orpulse duration of 12.5 μs plus decay time. Including the decay time, thepulse duration thus amounts to 50 μs (rise time 12.5 μs, decay time 37.5μs). However, current amplitudes up to 3 kA are possible within thescope of the invention. Moreover, rise times (pulse widths without decaytimes) of less then 50 μs can be realized for the current pulsesgenerated in non-resonant fashion.

In the curve 4 shown in FIG. 2, the steepness of the first current pulseof the curve 4 generated in non-resonant fashion corresponds to therespective initial steepness of the current curve 3 generated inresonant fashion (current pulse according to the prior art). However,the first current pulse of curve 4 generated in non-resonant fashion isterminated quickly (after approximately 12.5 μs) and a further currentof the curve 4 generated in non-resonant fashion is started soonthereafter (approximately 37.5 μs after the termination of the precedingcurrent pulse).

In the illustrated exemplary embodiment, all current pulses of the curve4 have the same pulse width. However, it is just as easily possiblewithin the scope of the invention to generate different pulse widthsand, resulting therefrom, different current amplitudes in a non-resonantfashion.

By terminating each current pulse 4 of the curve relatively quickly andstarting further current pulse soon thereafter, the required maximumcurrent maximum current drops considerably. In the illustrated exemplaryembodiment, the required maximum current drops from 9 kA to 1.5 kA,whereby maximum currents up to approximately 3 kA are possible withinthe scope of the invention. The high operating frequencies required forthis purpose can be realized unproblemmatically by IGBT and MOSFETmodules.

Due to the fact that the current pulses are generated in non-resonantfashion given the magnetic stimulation apparatus of the invention, theoutput voltage U_(CZ) (voltage curve 2) across the intermediate circuitcapacitor C_(Z) remains constant.

The non-resonant current pulses of the curve 4 shown in FIG. 2 lead tothe curve of the voltage across the stimulation coil L_(S) that is shownin FIG. 3. The current pulse widths of 12.5 μs lead to square-wavevoltage pulses that likewise exhibit a pulse width 12.5 μs and thatcorrespond in polarity to the current pulses of the curve 4 generated innon-resonant fashion. The voltage across the stimulation coil L_(S) isagain referenced to its peak value U/Û in FIG. 3.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

We claim as our invention:
 1. A magnetic stimulation apparatus fornon-invasively triggering action potentials, particularly in more deeplydisposed, neuro-muscular tissue of a patient, comprising a currentgenerating unit and at least one stimulation coil having terminalsconnected to an output of the current-generating unit, thecurrent-generating unit supplying current pulses generated innon-resonant fashion to the stimulation coil which cause saidstimulation coil to generate a magnetic field that directly depolarizesnerve cells to produce said action potential.
 2. A magnetic stimulationapparatus according to claim 1, wherein the current-generating unitcomprises at least one controllable power converter having at least onepower semiconductor switch with short switching times that can beswitched on and off.
 3. A magnetic stimulation apparatus according toclaim 1, wherein said current generator generates said current pulseswith an amplitude in a range from 1.5 kA through 3 kA.
 4. A magneticstimulation apparatus according to claim 3, wherein said currentgenerator generates said current pulses with a maximum rise time of 50μs.
 5. A magnetic stimulation apparatus according to claim 1, whereinsaid current generator generates said current pulses with a constantamplitude.
 6. A magnetic stimulation apparatus according to claim 1,wherein said current generator generates said current pulses with aconstant duration.
 7. A magnetic stimulation apparatus according toclaim 1, wherein said current generator generates said current pulseswith a constant amplitude and duration.
 8. A method for non-invasivelytriggering action potentials in neuro-muscular tissue of a patient,comprising the steps of: non-invasively placing a stimulation coil inrelation to a muscle; and supplying a current to said coil and therebycausing said coil to generate a magnetic field which directlydepolarizes nerve cells in said muscle to trigger action potentials insaid muscle.
 9. A method as claimed in claim 8 wherein the step ofsupplying a current to said stimulation coil comprises supplying currentpulses to said stimulation coil having an amplitude in a range from 1.5kA through 3 kA.
 10. A method as claimed in claim 9 comprising supplyingsaid current pulses with a maximum rise time of 50 μs.
 11. A method asclaimed in claim 9 comprising supplying said current pulses with aconstant amplitude.
 12. A method as claimed in claim 9 comprisingsupplying said current pulses with a constant duration.
 13. A method asclaimed in claim 9 comprising supplying said current pulses with aconstant amplitude and duration.